Solvation for Ions and Counterions:  Complementary Relation between Ion−Counterion and Ion−Solvent Interaction

The microscopic environments around a cation and its counterion were experimentally studied for methanol solutions of CsCl, LiCl, LiI, and LiClO 4 through mass spectrometric analysis of clusters generated from solutions. The change of the microscopic environments around cations and counteranions induced by the addition of 18-crown-6 were directly observed, which confirmed complementary relation between the ion−counterion and the ion−solvent interaction, as a representative solution property. For CsCl and LiCl solutions, by the addition of 18-crown-6, the solvated cation clusters (Cs + (CH 3 OH) m or Li + (CH 3 OH) m ) and the solvated anion clusters (Cl - (CH 3 OH) m ) were efficiently decreased and increased, respectively. By the complexation of the cation with the crown ether, the cation−methanol interaction was controlled, whereas the anion−methanol interaction was promoted through the indirect effect, that is, the decrease of the electrostatic influence of the cation on the anion. Accordingly, the solvated anion clusters formed in the presence of crown ether reflect the intrinsic anion−methanol interaction. The intrinsic solvation for Cl -, I -, and ClO 4 - were compared by the measurements of their lithium salts in methanol including 18-crown-6. The observed solvated anion clusters had good relation with the free-energy change for solvation of anions by methanol.